In recent years, with the progress of semiconductor techniques, power converters treating large power have been of practical use, and, a microwave oven or the like which uses a resonant type high frequency inverter power supply (power converter) of about 20 kHz to 50 kHz, has been put into practical use. In power converters having a semiconductor switching element, over-voltage protection means for preventing breakdown in a withstand voltage of the semiconductor switching element is an essential constituent element.
FIG. 6 is a partial circuit configuration diagram illustrating an example of the over-voltage protection means of the semiconductor switching element shown in an inverter power supply in the related art (for example, refer to Patent Document 1).
In FIG. 6, the inverter power supply 30 is provided with a forced commutation circuit 31 having a withstand voltage quantity set to be lower than a withstand voltage quantity of a semiconductor switching element 3. In a case where an over-voltage equal to or greater than the set voltage occurs across terminals of the semiconductor switching element 3, information regarding the over-voltage is fed back (input) to a base (control terminal) of the semiconductor switching element 3, and the semiconductor switching element 3 is changed from an OFF state to an ON state, thereby suppressing the over-voltage.
Since the inverter power supply 30 shown in the figure is of a resonant type, and the semiconductor switching element 3 is operated at a voltage (collector voltage) across terminals of about 600 V to 650 V, a semiconductor switching element having a withstand voltage of 1000 V is generally selected. Therefore, the set voltage of the forced commutation circuit 31 is set to about 800 V to 900 V in consideration of overshoot at the time of transition in addition to the collector voltage.